AL8807 datasheet(13/21 Pages) DIODES

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AL8807

HIGH EFFICIENCY LOW EMI 36V 1A BUCK LED DRIVER

AL8807

Document number: DS35281 Rev. 4 - 2

13 of 21

www.diodes.com

June 2012

Applications Information (cont.)

The following equations can be used as a guide, with reference to Figure 1 - Operating waveforms.

Switch ‘On’ time

Switch ‘Off’ time

()

AVG

LED

−

()

AVG

LED

OFF

Where:

L is the coil inductance (H)

rL is the coil resistance (Ω)RS is the current sense resistance (Ω)

Iavg is the required LED current (A)

ΔI is the coil peak-peak ripple current (A) {Internally set to 0.3 x Iavg}

VIN is the supply voltage (V)

VLED is the total LED forward voltage (V)

RSW is the switch resistance (Ω) {=0.5Ω nominal}

VD is the diode forward voltage at the required load current (V)

Thermal Considerations

For continuous conduction mode of operation, the absolute maximum junction temperature must not be exceeded. The

maximum power dissipation depends on several factors: the thermal resistance of the IC package

θJA, PCB layout, airflow

surrounding the IC, and difference between junction and ambient temperature.

The maximum power dissipation can be calculated using the following formula:

PD(MAX) = (TJ(MAX) − TA) / θJA

where

TJ(MAX) is the maximum operating junction temperature,

TA is the ambient temperature, and

θJA is the junction to ambient thermal resistance.

The recommended maximum operating junction temperature, TJ, is 125°C and so maximum ambient temperature is

determined by the AL8807’s junction to ambient thermal resistance,

θJA and device power dissipation.

θJA, is layout dependent and package dependent; the AL8807W5’s θJA on a 25x25mm single layer PCB with 1oz copper

standing in still air is approximately 250°C/W (160°C/W on a four-layer PCB).

The maximum power dissipation at TA = 25°C can be calculated by the following formulas:

PD(MAX) = (125°C − 25°C) / (250°C/W) = 0.4W for single-layer PCB

PD(MAX) = (125°C − 25°C) / (160°C/W) = 0.625W for standard four-layer PCB

Figure 35, shows the power derating of the AL8807W5 on two (one single-layer and four-layer) different 25x25mm PCB with

1oz copper standing in still air and the AL8807MP on an FR4 51x51mm PCB with 2oz copper standing in still air.

-40 -25 -10

20 35 50 65 80 90 110 125

AMBIENT TEMPERATURE (°C)

Fig. 35 Derating Curve for Different PCB

1600

1400

1200

1000

800

600

400

200

SOT25_1 Layer

MSOP-8EP

SOT25_4 Layer

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